The Effects of Compensation Injections on Tunnels

Siew-Wei Lee, Cambridge University
Geotechnical Engineering Group


This dissertation investigates the effects of compensation injections on segmented tunnel linings using site observation data, as well as centrifuge and numerical (finite element analysis) modelling in dry sand.

The site data revealed that good tunnelling practice resulted in surface settlements of less than l5mm and a mean volume loss of 0.7%. Numerical analyses of compensation grouting suggested that grouting over a wider area could reduce the angular distortion suffered by an overlying structure.

Seven strip injection tests revealed that surface heave was dependent on the ratios of grouting pressure over overburden and grouting depth over width. A mechanism of uplift was proposed to predict the maximum uplift resistance. The segmental tunnel lining displacement was found to be dependent on the ratios of grouting pressure over overburden, grouting thickness over depth and grouting separation (distance to the tunnel crown) over width. A simple grouting pressure transfer mechanism was proposed to describe the data of maximum tunnel crown displacement in terms of grout mg geometries, pressure and volume.

Three multiple-point simultaneous injection tests suggested that the smaller the coverage ratio the larger the surface heave, while there was less displacement of the tunnel lining for a given injection volume. Seven multiple-point sequential injection tests showed that the soil compression or stiffening caused by previous injections and the higher grouting pressure of the subsequent injection created more surface heave and lining displacement.

Three multiple-point asymmetrical sequential injection tests revealed the effect of an increase in grouting pressure, as grouting advanced towards the tunnel. The rotation of lining segments in different directions, subjected to asymmetrical grout loading, could increase the stress and strain at the lining joints. This became more profound if the asymmetrical injection was of the simultaneous injection type.

The numerical modelling showed that predictions based on the critical state model was highly dependent on the pre-consolidation pressure or over-consolidation ratio. The Mohr-Coulomb model was able to predict the peak strength of soil, provided that the soil friction and dilation angles could be determined. The numerical modelling illustrated different modes of soil shearing when subjected to injection, as well as stress path reversal undergone by soil elements in the sequential injection tests.

Finally, it was found that multiple-point simultaneous injection at a smaller coverage ratio was the best option to create surface heave while minimising the tunnel lining displacement for a given injection volume in dense sand. Some recommendations were made to relate these research findings to practice.

Keywords: compensation grouting, compression, dilation, grouting geometries, grouting pressure, heave, injection volume, numerical modelling, sequential injection, settlement, simultaneous injection, stress path, strip injection, tunnel lining displacement.